1. Field of the Invention
This invention relates to an electrochemical cell having a nonaqueous electrolyte and, particularly, to a rechargeable lithium ion cell. The activating electrolyte is provided with a polymer additive comprised of repeating vinyl units joined to one or more heterocyclic amine moieties. This improves the cycle life and shelf life of the cells.
2. Description of Related Art
The use of rechargeable lithium-ion secondary cells in portable electronic equipment such as cell phones, PDAs, and laptop computers, and in small medical devices such as neurostimulators and insulin pumps has continued to increase in recent years. In the medical device industry, the requirements for these cells imposed by the Food and Drug Administration and by the device manufacturers are very stringent. In particular, it is important that these cells have a stable and predictable shelf life prior to use, so that they can receive an initial charge and be reliably placed into service. Additionally, they must be capable of being repeatedly discharged and recharged in a predictable manner during their life cycles.
Rechargeable lithium-ion secondary cells are generally comprised of a negative electrode (anode), a positive electrode (cathode) and an intermediate separator. The typical fabrication of such a cell involves winding the anode, cathode and separator to form a jellyroll, or sequentially arranging the anodes and cathodes which are provided as plates to form a cell stack. The jellyroll or cell stack is then housed in a metal (titanium or titanium alloy, stainless steel, aluminum) casing or aluminum laminated packaging film, and then filled with electrolyte. This is followed by sealing the casing by welding, or by sealing the packaging film using suitable means such as a foot sealer.
There has been considerable investigation into the use of electrolyte additives to improve cycle life and shelf life of rechargeable cells such as lithium ion cells. In particular, optimized electrolyte compositions are important to obtain lithium ion cells having improved cycle performance and shelf life. Most of the reported additives are comprised of monomers, which are intended to beneficially modify the solid electrolyte interface (SEI) layer on the anode surface during charging the cells. The formation of a surface film is unavoidable for alkali metal anodes, and in particular, lithium metal anodes and lithium intercalated carbon anodes due to their low potential and high reactivity towards organic electrolytes. Once formed, the SEI layer prevents further reaction of the electrolyte on the anode surface, thereby improving the cycle performance of the cells.
Ethylene carbonate is known to form a stable SEI layer on an anode surface at approximately 0.8V vs. Li/Li+. The SEI layer influences several important aspects of battery performance such as cycle life, power capability and self-discharge rate or shelf life. Preferably, the SEI layer formed on the anode surface is relatively thin and of a uniform thickness. If it is not uniform, a relatively thick SEI layer will be formed on the electrode surface due to lithium deposition on the anode. This results in poor cycle life and higher self-discharge rate due to consumption of additional electrolyte on the electrode surface.
Various electrolyte additives have been disclosed which form a passivation layer on an electrode, stabilize the passivation layer, improve thermal stability of the electrolyte and cell, and improve overcharge safety. Although the cyclic performance and shelf life of lithium ion cells has been improved significantly by using organic additives in electrolytes, there remains a considerable need for further improvements in order to satisfy the increasingly stringent performance requirements in new cell applications.